The inventions of Assignee's pressure isolation tools that provide full-bore access to a casing of the well have revolutionized the well stimulation industry. Full-bore isolation tools seal or “pack off” at a fixed-point above a bit guide in a seal bore of a tubing head spool that supports a production casing in the well, and permit full bore access to the production casing.
A fixed-point packoff element provides a high pressure seal in the tubing head spool seal bore. The full-bore access pressure isolation tools permit stimulation fluid to be pumped into a well at pressures of 15,000+ psi and rates of 300+ bbl/minute. As well, the fixed-point packoff element permits any downhole tool to be run into the casing so that multiple production zones can be stimulated without unrigging frac irons or the full-bore pressure isolation tool.
Since stimulation fluids are commonly pumped into the well at fluid pressures of at least 10,000 psi, the packoff element must provide a very reliable high pressure seal. In order to do so, the packoff element must have an outer diameter that is not more than about 0.005″-0.030″ smaller than an inner diameter of the tubing head spool seal bore. As is well understood in the art, an exact inner diameter of a tubing head spool seal bore is not always known with certainty. Consequently, full-bore pressure isolation tools have been rigged up as follows: a packoff element is selected based on a best knowledge of an inner diameter of the tubing head spool seal bore; the packoff element is run it and set; frac irons are hooked up to the full-bore pressure isolation tool and the well is pumped up to test pressure; a tubing head spool side port is monitored for pressure leaks; and, if the packoff element does not contain the test pressure, the test pressure is bled off, the frac irons are disconnected, the pressure isolation tool is removed from the wellhead and the packoff element is changed for a next larger diameter, typically about 0.015″ larger. The entire process is then repeated. This can be a time consuming and expensive process.
There therefore exists a need for a fixed-point packoff element with a primary seal test capability to permit primary seal testing prior to rigging up the pressure isolation tool for a well stimulation operation.